Method of impregnating porous tungsten and resulting article



Jan. 13, 1970 w. E. MCKEE 3,489,602

METHOD OF IMPREGNATING POROUS TUNGSTEN AND RESULTING ARTICLE Filed May 25, 1965 Pazoaf Twas/2W 4er/a5 United States Patent Oiice METHOD F IMPREGNATIN G POROUS TUNGSTEN AND RESULTING ARTICLE William E. McKee, Woodland Hills, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed May 25, 1965, Ser. No. 458,613 Int. Cl. B44d 1 34; B22f 7/00 U.S. Cl. 117-213 9 Claims ABSTRACT OF THE DISCLOSURE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Sec. 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2'475').

This invention relates to a method for depositing or impregnating metals into porous tungsten and particularly for impregnating porous tungsten with refractory and noble metals to produce a gas-permeable, high temperature resistant coating upon the porous tungsten.

Refractory metals such as rhenium, molybdenum and tantalum, the noble metals, gold and silver, and the platinum metals, platinum, osmium, ruthenium, irdium and rhodium have been deposited upon tungsten lby the `use rof a sputtering process. However, the sputtering process is complex, requiring the use of high vacuum and high voltage sputtering equipment. Furthermore, by the use of the sputtering process, the internal surfaces of the surface pores of tungsten are not coated with the deposited metal and a layer of impregnant is deposited upon the outer surface only -of the porous tungsten. Moreover, the sputtering process often codeposits surface impurities from the vacuum environment on the porous tungsten.

Accordingly, it is a primary object of this invention to provide a low cost, convenient and eicient method for impregnating porous tungsten with high purity metals.

Another object of this invention is to provide a convenient method for depositing a thin, uniform, durable layer of gas-permeable metal upon porous tungsten.

A further object of this invention is to provide a method for impregnating porous tungsten with metals so that the internal surfaces as well as the external surfaces of the outer layers of pores of the porous tungsten are impregnated with porous metal.

Additional objects of the invention will become apparent from the following description, which is given primarily for purposes of illustration, and not limitation.

The accompanying drawing is a fl-ow sheet illustrative of an application of a process or method as described herein.

Stated in general terms, the objects of the invention are attained by preparing a porous tungsten article or body for impregnation by wet hydrogen ring at about 1000 to about 1200 C. for about an hour, impregnating the fired article by submergng it in a suitable solution containing a soluble compound of the impregnating 3,489,602 Patented Jan. 13, 1970 metal, drying the impregnated article in air after recovery thereof from the impregnating solution, heating the dried article, or heating it in hydrogen at about 900 to about 1000 C. to reduce the impregnating metalcontaining material to the free metal, and repeating the above-recited impregnation, drying and hydrogen tiring steps until the desired amount of metal has been impregnated into the tungsten article. The compound of the metal to be impregnated should be dissolved in a solvent which is volatile aud non-reactive with tungsten. The compound should be susceptible to reduction 'by heat or hydrogen t-o the metallic state.

A solution suitable for impregnating porous tungsten articles, bodies or electronic parts with irdium, or other impregnating metal, is made of a metal compound in a suitable, non-reactive volatile solvent such as, concentrated ammonia, alcohol, water, nitric acid, hydrochloric acid, or tetrahydrofuran. The impregnant metal compound used should contain no other undesirable, nonvolatile metallic elements, lor chemical groups, such as sulfate, carbonate or phosphate, which might react with or become a part of the tungsten surface. The halide and nitrate compounds of metals and ammonia and nitroso complexes of metal halides are suitable for making such a solution, although others can be used.

Other examples of suitable solutions are composed of ruthenium trichloride, osmium trichloride, platinum chloride and palladium chloride in hydrochloric acid solutions, rhenium pentachloride in absolute alcohol or tetrahydrofuran, rhodium nitrate in water or dilute nitric acid.

A typical solution is made by adding 0.1 gram of IrCl3 to 3 ml. of concentrated ammonia. The solution is warmed slightly, but not allowed to boil, to dissolve a occulent precipitate that forms. It is then cooled and tightly stoppered to prevent loss of ammonia.

Porous tungsten parts are prepared for impregnation by wet hydrogen firing at 1000-12001" C. for one hour, as indicated at 10. The cooled tungsten is immersed slowly in the ammonia solution in a manner which will permit the entrapped gases in the pores of the tungsten to be displaced Iby liquid, as indicated at 11.

An alternative impregnating method is to spray the solution on to the 'part which can 4be heated to 50 C. or more to hasten evaporation of the solvent. This method minimizes the amount of metal which is deposited in the interior of the porous tungsten. The tungsten is then placed in an air oven at C. for approximately 30 minutes or until dry, as indicated at 12. If desired, the dried tungsten may be immersed briefly or sprayed a second time and dried again.

The impregnated irdium compound is reduced to the metallic state Vby heating the porous tungsten in a noncarbonaceous hydrogen atmosphere at: 900 to 1000 C. for ten minutes, or more, as indicated at 13. If organic solvents have been used, it is often desirable to use wet hydrogen to reduce changes of contamination by the formation of tungsten carbide. The above impregnating process, followed by drying and hydrogen ring, can be repeated until the desired amount of irdium has been impregnated into the tungsten, as indicated at 14. If an organic solution has been used, it is generally advisable to carry out the 900 to 1000 C. firing in wet hydrogen.

Normally it would be expected that the irdium would be deposited more or less uniformly throughout the pores of the porous tungsten. However, it was found that a major part is deposited as a continuous layer over the tungsten particles which form the outer surface and the outer layer or two of pores without substantially reducing the porosity of the surface. In the" case of the irdium example, six impregnations with the aforementioned solution will deposit a layer about 0.5 micron thick on the surface and in the outer layer of pores corresponding to about 0.3% by weight of iridium. Eleven impregnations produce a surface layer about 1 micron thick with a total addition of about 0.5% by weight of iridium.

After every five or six impregnations, the porous tungsten should be heated to 1500 C. in dry hydrogen for 2 to 4 hours to bond the deposited layer to the tungsten substrate. Tests indicate that the surface layer of about a micron thickness remains in place for at least 25 hours at 1500 C. in hydrogen and 4 hours at 1800 C. in vacuum.

The exact composition of the iridium layer is not known, but it undoubtedly contains some tungsten as an alloy as its crystal structure corresponds more closely to a close-packed hexagonal rather than cubic as in the case of pure iridium and tungsten. This iridium coating has been used to modify the cesium ionization characteristics of a porous tungsten ionizer on a cesium ion beam. Iridium has a higher electron work function than tungsten. Thus an iridium surface emits fewer neutral cesium atoms and prolongs the useful life of a cesium vapor ionizer. An additional benefit results from the fact that sintering a fine pore ionizer, prepared by the method of the invention, over long periods of time may be inhibited by the presence of traces of iridium throughout the body of the porous tungsten.

Obviously many other modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention can be practiced otherwise than is specifically described.

What is claimed is:

1. The method of impregnating the internal and ex.- ternal surfaces of a prefabricated porous tungsten article with a metal selected from the group consisting of refractory metals, noble metals, and platinum metals comprising the steps of preparing the porous tungsten article for impregnation by wet hydrogen firing said article in the order of l000 C. to 1200 C. for about an hour, impregnating the said tired article with a non-reactive volatile 4 solvent solution impregnant of a heat or hydrogen reducible compound of said metal containing no undesirable non-volatile metallic element, or chemical group capable of reacting with or becoming a part of the tungsten surface, drying the impregnated tungsten article, and heating the impregnated tungsten article to a temperature eiecting reduction of said metal compound to free metal deposited upon the porous tungsten article surface without substantially reducing the porosity of the surface.

2. The process of claim 1 wherein the impregnated red tungsten article is heated in a hydrogen atmosphere.

3. The process of claim 2 wherein the impregnant is a solution containing a compound of a refractory metal.

4. The process of claim 2 wherein the impregnant is a solution containing a compound of a noble metal.

5. The process of claim 3 wherein the impregnant is a solution containing a compound of a platinum metal.

6. The method of claim 1 wherein the impregnant is a concentrated ammonia solution.

7. The method of claim 1 wherein the impregnant is a concentrated ammonia solution containing an iridium salt and the reduction temperature is on the order of 900 t0 1000 C. eifected in a hydrogen atmosphere.

8. The product produced by the process of claim 1.

9. The method of claim 1 including the added step of further impregnating the said impregnated tungsten article with said solvent solution after the drying step, or the reducing step, or after each said step.

References Cited UNITED STATES PATENTS 656,651 8/1900 Markey 117-227 1,018,502 2/1912 Just et a1 75-224 X 1,390,243 9/1921 Laise 75-224 X 2,121,637 -6/1938 Lemmers et al. 117-49 X 2,215,645 9/ 1940 Iredell et al. 75-200 2,282,097 5/1942 Taylor 117-227 X 2,602,757 7/1952 Kantrowitz 117-227 X 2,853,401 9/1958 rMackiw et al. 117-13-0 X 3,147,154 9/1964 Cole et al 117-130 X 3,265,526 8/1966 Beer 117-50 0 ALFRiED L. LEAVITT, Primary Examiner J. R. BATTEN, In., Assistant Examiner ggg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,489, 602 Dated January 13, 1970 Inventor(s) W. E. McKee It is certified that: error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. l, Col. 2, line 57, Col. 4, line l2, "3"

SIGNED 'IIND SEALED MAI 121970 iSEAL) Attest:

Edward M. Fletcher, In WILL-Mis; SGHUYLER, Commissioner of Paten' mening officer 

